Maliks Muwatta Book 2, Hadith Number 37.
Section : On Wudu in General.
Yahya related to me from Malik that he had heard that the Messenger of Allah, may Allah bless him and grant him peace, said, “Try to go straight, although you will not be able to do so. Act, and the best of your actions is the prayer. And only a mumin is constant in his wudu.”